In the foundry industry, one of the procedures used for making metal parts is “sand casting”. In sand casting, disposable foundry shapes, e.g. molds, cores, sleeves, pouring cups, coverings, etc. are fabricated with a foundry mix that comprises a mixture of a refractory and an organic or inorganic binder. The foundry shape may have insulating properties, exothermic properties, or both.
Foundry shapes such as molds and cores, which typically have insulating properties, are arranged to form a molding assembly, which results in a cavity through which molten metal will be poured. After the molten metal is poured into the assembly of foundry shapes, the metal part formed by the process is removed from the molding assembly. The binder is needed so the foundry shapes do not disintegrate when they come into contact with the molten metal. In order to obtain the desired properties for the binder, various solvents and additives are typically used with the reactive components of the binders to enhance the properties needed.
Foundry shapes are typically made by the so-called no-bake, cold-box processes, and/or heat cured processes. In the no-bake process, a liquid curing catalyst is mixed with an aggregate and binder to form a foundry mix before shaping the mixture in a pattern. The foundry mix is shaped by compacting it in a pattern, and allowing it to cure until it is self-supporting. In the cold-box process, a volatile curing catalyst is passed through a shaped mixture (usually in a corebox) of the aggregate and binder to form a cured foundry shape. In the heat cured processes the shape mixture is exposed to heat which activates the curing catalyst to form the cured foundry shape.
There are many requirements for a binder system to work effectively. For instance, the binder typically has a low viscosity, be gel-free, and remain stable under use conditions. In order to obtain high productivity in the manufacturing of foundry shapes, binders are needed that cure efficiently, so the foundry shapes become self-supporting and handleable as soon as possible.
With respect to no-bake and heat cured binders, the binder typically produces a foundry mix with adequate worktime to allow for the fabrication of larger cores and molds. On the other hand, cold-box binders typically produce foundry mixes that have adequate benchlife, shakeout, and nearly instantaneous cure rates. The foundry shapes made with the foundry mixes using either no-bake, cold-box or heat cured binders typically have adequate tensile strengths (particularly immediate tensile strengths), scratch hardness, and show resistance to humidity.
One of the greatest challenges facing the formulator is to formulate a binder that will hold the foundry shape together after is made so it can be handled and will not disintegrate during the casting process,1 yet will shakeout from the pattern after the hot, poured metal cools. Without this property, time consuming and labor intensive means must be utilized to break down the binder so the metal part can be removed from the casting assembly. Another related property required for an effective foundry binder is that foundry shapes made with the binder must release readily from the pattern. Casting temperatures of poured metal reach 1500° C. for iron and 700° C. for aluminum parts.
The flowability of a foundry mix made from sand and an organic binder can pose greater problems with respect to cold-box applications. This is because, in some cases, the components of the binder, particularly the components of phenolic urethane binders, may prematurely react after mixing with sand, while they are waiting to be used. If this premature reaction occurs, it will reduce the flowability of the foundry mix and the molds and cores made from the binder will have reduced tensile strengths. This reduced flowability and decrease in strength with time indicates that the “benchlife” of the foundry mix is inadequate. If a binder results in a foundry mix without adequate benchlife, the binder is of limited commercial value.
In view of all these requirements for a commercially successful foundry binder, the pace of development in foundry binder technology is gradual. It is not easy to develop a binder that will satisfy all of the requirements of interest in a cost-effective way. Also, because of environmental concerns and the cost of raw materials, demands on the binder system may change. Moreover, an improvement in a binder may have some drawback associated with it. In view of these requirements, the foundry industry is continuously searching for new binder systems that will reduce or eliminate these drawbacks.
Although there has been tremendous progress in the development of foundry binder systems, there are still problems associated with the use of organic binder systems. Of particular concern are problems associated with the by-products that are generated from the actual decomposition of the binders. These problems include casting defects such as warpage, scabs, erosion, lustrous carbon, carbon pickup, and rattails caused by the expansion of the sand and loss of strength of the binder. Various additives such as iron oxides and various blends of clays, sugar, and cereals are used to help to minimize or eliminate many of these defects. However, the use of specialty sands and sand additives only addresses the types of defects associated with the expansion of the sand and cooling of the metal.
Additionally, the use of these additives can cause other problems such as reduced strengths within the core or mold, gas defects and smoke caused by the additional gasses coming from the organic additives. Furthermore, additives can affect the ability of the binder to create a strong core, mold, or other shapes because they either soak up some of the binder or introduce large amounts of fine particles which add to the surface area that the binder needs to coat which, either way, effectively reduces the strength of the overall mixture. The use of an additional binder can overcome the strength losses caused by the use of traditional additives but this can in turn increase the presence of defects related to the decomposition products of the binder system such as gas defects, smoke, lustrous carbon, and carbon pickup in the metal. Without the additional binder to compensate for the loss of strength when using the traditional additives, other defects such as erosion, warpage, scabs, and rattail defects can be exacerbated.
Examples of foundry shapes that may be required to have exothermic properties include, for example, sleeves, floating coverlids, and coverings or pads for other parts of the casting and/or gating system. Exothemmic foundry mixes used to make these foundry shapes comprise a refractory, an oxidizable metal, a compound that is a source of oxygen, and typically an initiator for the exothermic reaction. Exothermic foundry mixes are also used for materials such as powdered hot toppings and other materials where a bonding agent is not applied and there is no curing of the material.
Foundries use exothermic materials and shapes having exothermic properties to keep the molten metal, used to prepare metal parts, in its liquid state longer, so that premature solidification of the metal does not occur. Although conventionally used exothermic materials and shapes having exothermic properties are effective, there is a need to provide new materials that impart improved exothermic properties to the foundry materials and shapes having exothermic properties. In particular, there is a need for exothermic foundry mixes that provide improved exothermic properties without adversely affecting other exothermic properties. There is also a need to provide exothermic foundry mixes that allow the formulator to customize the formulation for the preparation of specific metal parts.
More specifically, it is important to control the amount of energy that it takes to start the exothermic reaction. Ideally, one wants to use the least amount of energy to start the exothermic reaction needed for the particular application, yet maximize the burn temperature, total amount of energy released, and maintain the exothermic material burn as hot as possible for as long as possible.
If one uses the exothermic foundry mixes known in the prior art, there is a limit as to how the formulator can customize the exothermic foundry mixes for the preparation of specific metal castings. For instance, if the formulator wants the exothermic reaction to initiate using less energy, then you have to use a finer particle size of aluminum. However, if the formulator does this, then the duration of the exothermic reaction and the maximum temperature reached are adversely affected. On the other hand, if the formulator uses a larger particle size of aluminum to increase the duration of the exothermic reaction and increase the maximum temperature, the energy to ignite is higher. Because of this, foundries often use a blend of two different particle sizes of aluminum, but it is apparent that this result is not completely satisfactory.